Exhaust gas purification device for an internal combustion engine

Abstract

The exhaust gas purification device includes a NO.sub.x absorbent disposed in an exhaust gas passage of an internal combustion engine. The engine is mainly operated at a lean air-fuel ratio. The NO.sub.x absorbent absorbs NO.sub.x in the exhaust gas from the engine when the exhaust gas flowing into the NO.sub.x absorbent is at a lean air-fuel ratio, and releases the NO.sub.x absorbed therein and reduces it when the air-fuel ratio of the exhaust gas becomes a rich air-fuel ratio. A control circuit is provided for controlling the operating air-fuel ratio of the engine. When the ignition switch of the engine is turned off, the control circuit continues the engine operation for a predetermined period before terminating the engine operation. During this engine operation, the operating air-fuel ratio of the engine is controlled at a rich air-fuel ratio to supply exhaust gas with a rich air-fuel ratio to the NO.sub.x absorbent. Therefore, NO.sub.x absorbed and held therein when the ignition switch is turned off is released and reduced by the rich air-fuel ratio exhaust gas from the engine. Since the NO.sub.x absorbent is maintained at the condition where no NO.sub.x remains in the NO.sub.x absorbent after the engine has stopped, unreduced NO.sub.x is not released from the NO.sub.x absorbent even if the NO.sub.x absorbent is cooled after the engine has stopped.